1 Introduction

During the past few years studies of long term optical light curves of blazars have indicated that several of them show periodic variations on time scales of few years (e.g. OJ 287, Sillanpää et al. 1988; Kidger et al. 1992; BL Lac, Fan et al. 1998a; ON 231, Liu et al. 1995; Tosti et al. 1998). These periodicities have been explained by a variety of models; e.g. binary black hole (Sillanpää et al. 1988; Valtonen & Lehto 1997), precessing jet (Katz 1997) and rotating helical jet (Villata et al. 1998; Villata & Raiteri 1999). Occasionally, indications of recurring time scales from a few days to weeks and months have been reported (e.g. OJ 287 9.3 days, Sillanpää 1991; 3C 66A, 65 days, Lainela et al. 1999; S5 0716+714 from one to seven days, Wagner et al. 1996). The reasons for these shorter time scale variations are still unclear (e.g. Wagner & Witzel 1995). Confirmation and understanding of all these preferred timescales will provide new tools in revealing the mystery of BL Lac objects.

Blazar OJ 287 is one of the best observed extragalactic objects. It has been observed for over 100 years, providing us a very good historical light curve (see Takalo 1994 for a review of the earlier observations). Based on the historical light curve Sillanpää et al. (1988) concluded that OJ 287 shows large optical outbursts with a period of 11.6 years (see also Kidger et al. 1992). They explained these periodic outbursts with a binary black hole model, in which the brightenings are due to enhanced inflow from the larger accretion disk into the primary black hole, caused by strong perturbations every time the secondary black hole is in proximity of the pericentre. Sillanpää et al. (1988) predicted that the next outburst would occur during the fall 1994. In order to verify this outburst the OJ-94 Project was created in 1993.

In the next section we give first a short description of the OJ-94 project. Section 3 will describe briefly the observations and data reductions, followed by the results in Sect. 4. Conclusions will be given in Sect. 5.